1. Field of the Invention
The subject invention relates to oscillators and more particularly to apparatus that eliminates the effect of feedback reflections on the output of a master oscillator, and, more particularly, a laser master oscillator.
2. Description of Related Art
A small amount of output power reflected back into an oscillator can have a large influence on the output of the oscillator. With respect to a laser master oscillator, the feedback is equivalent to coupling another resonant cavity to the existing oscillator, and the feedback will interfere with the circulating wave within the oscillator. This can have a devastating impact on the performance of a laser master oscillator/power amplifier (MOPA) system, as a small amount of power will be reflected back into the oscillator from the amplifier, and the resulting output fluctuations will be amplified. In a multisection amplifier, the impedance change on moving from amplifier section to section gives rise to a reflection at each section, so that there are multiple feedback waves, each with its own phase relation to the original oscillator.
This effect is further exaggerated in a frequency chirped MOPA, as the phase relationships among the master oscillator and the reflected waves are constantly changing. The output power therefore becomes an erratic function of the frequency of the oscillator. As the rate of frequency chirp and the chirp repetition rate increase, the oscillator power fluctuations interact with the resonant cavity decay time and with the time constants of the gain medium. This interaction distorts the fluctuations so that they are no longer even determined by the instantaneous oscillator frequency and the phase shifts of the feedback reflections but also by the time history of the oscillator output.
Prior art techniques to isolate an oscillator from back reflections in an amplifier are (1) an extreme effort to eliminate reflections, (2) polarization isolation with Faraday rotators, and (3) polarization isolation with waveplates. These techniques have a number of disadvantages.
(1) While spurious reflections should be minimized, the complexity and expense can be prohibitive, and even then may be unsuccessful.
(2) Faraday rotation is the property some materials have of rotating the plane of polarization of a transmitted beam when the material is subjected to a magnetic field. The return trip through the rotator does not unrotate the effect, but doubles the rotation. By using a 45.degree. rotator, the feedback beam polarization is rotated 90.degree. from the original beam and can be blocked with a polarizer. Faraday isolators are often used for wavelengths shorter than 1 .mu.m, but there are no good Faraday rotator materials in the longer infrared, such as the CO.sub.2 laser wavelength of 10.6 .mu.m. As an example of the difficulties, the CO.sub.2 MOPA at the Lincoln Laboratory Firepond facility requires a 70 kgauss superconducting magnet for its Faraday rotator.
(3) Waveplate isolation works by using a quarter-waveplate to convert linearly-polarized light into a circularly-polarized waveform. Reflections are then circularly polarized in the opposite sense, and the waveplate converts them to linearly-polarized light orthogonal to the original, which can then be rejected by a polarizer. However, circular polarization between the oscillator and the reflectors is impractical in many systems.
In a MOPA system, the amplifier may have, for example, eight total-internal-reflection folds, each of which introduces a large phase shift between the horizontal and vertical components. These phase shifts will convert circular polarization to various elliptical polarization states, which then will not be converted back to orthogonal linear polarization at the waveplate. Also, the amplifier has a different transmission for horizontal and vertical polarization which affects its net gain for circular polarization as well as turning circular polarization into elliptical.
Neither Faraday rotation nor waveplate isolation can protect against scattered light, as it is not specially polarized.